This is a double-blind placebo controlled pilot study to determine if vitamin D supplementation in hemodialysis (HD) patients will improve physical function and cognition. HD patients have a high prevalence of vitamin D deficiency, cognitive impairment, and physical impairment. Despite standard clinical care with active IV vitamin D during dialysis, HD patients still have markedly low levels of nutritional or dietary vitamin D. IV treatment with vitamin D during dialysis is aimed at treating HD related bone disease. Recent literature shows that oral or nutritional vitamin D has multiple extra-skeletal effects including improvement in cognition and physical function. In this study, the investigators plan to administer oral vitamin D to vitamin D deficient HD patients already receiving standard care with IV vitamin D therapy. Patients will be randomized to receive either placebo or 50,000 IU of vitamin D (cholecalciferol) weekly for 6 months. The investigators' specific aims are to 1) Assess the benefits of correcting nutritional vitamin D deficiency on cognitive and physical function in HD patients receiving routine standard of care, 2) Assess the feasibility of recruiting HD patients for this study, and 3) Evaluate the proposed regimen for safely and effectively increasing nutritional vitamin D levels with oral supplementation. The investigators anticipate that correction of nutritional vitamin D deficiency to optimal levels will improve the high prevalence of cognitive impairment and physical impairment in this population. These results will be used as evidence to support a larger study aimed at treating nutritional vitamin D deficiency in all patients receiving HD. These results may also contribute to a change in current guidelines which place little emphasis on the monitoring and treatment of nutritional vitamin D levels in HD patients. These results are important for the Veteran dialysis population since many of them are required to perform high-level cognitive tasks such as management of complex medical regimens and physical tasks such as orchestrating independent transportation to and from HD sessions and multiple physician appointments.